I keep Honeybees north of Pontiac, and I often find garters in and under my equipment.
I occasionally see a tail disappearing into a rock pile, but for the most part they are better at hiding than I am at spotting them.

Could you place some boards discreetly around the area you want to find them in? Snakes like to hide beneath boards and logs and you may find that somebody has taken up residence if you check after a few days.